Traditionally in anaesthesia, the respiratory gas administered to a patient is formed in a flow mixer. This is typically linked with a pressurized hospital gas supply system (O.sub.2 and N.sub.2 O), the supply pressures fluctuating within the range of 3-8 bars. The gases are mixed at a desired ratio by regulating the flows by means of adjustable throttle elements and gas-related flowmeters. Downstream of throttle elements and flow measurement the flows are combined. By this time, the pressure has dropped close to ambient external pressure. The compound gas is delivered to an anaesthesia vaporizer for vaporizing anaesthetic vapour therein at a desired concentration. The gas flowing out of the vaporizer is supplied to a patient as alveolar air.
The operation of a conventional anaesthesia vaporizer is based on natural vapour pressure. A fresh gas flow, which thus does not contain the vapour of a vaporizable liquid, is delivered into a liquid container. Here the gas is saturated according to the vapour pressure of a liquid with an anaesthetic vapour. With traditional anaesthetic liquids the boiling point is approximately 50.degree. C. and the vapour pressure at room temperature approximately 25% of the external pressure which is also the concentration of a liquid discharging from the liquid container. Since the desired concentration for a mixture discharging from a vaporizer is about 0.5-5%, the gas flow must be diluted. This is effected by passing just part of the overall flow through a liquid container. By adjusting the ratio of flows passing through and by the container it is possible set the anaesthetic concentration of a gas discharging from the vaporizer as desired. The vaporizers have been calibrated to a diluted anaesthetic concentration.
A new liquid is being introduced in anaesthesia and it termed as 2-(difluoromethoxy)-1,1,1,2-tetrafluoroethane or generally called desphlurane. In terms of its technical qualities relating to vaporization, this differs from traditional liquids in its boiling point, which at normal pressure is 23.5.degree. C. With the boiling point matching the temperature of a normal operating environment, the traditional vaporizing techniques lead to uncontrollable anaesthetic concentration. In the worst case, with the liquid boiling, the anaesthetic vapour discharges into the alveolar air of a patient in a pressurized state, which may quickly result in fatally dangerous concentrations.
In order to control vaporization, an active effort has been made to maintain the liquid temperature above the boiling point. This can be achieved by thermostating the liquid to a desired temperature.
A method based on heating is widely described in U.S. Pat. No. 4,881,541. In this method, the amount of anaesthetic is metered by regulating the flow by means of a needle valve and a rotameter unlike in traditional vaporizers, wherein the regulation was effected by means of concentration. The method has several drawbacks. Since the desphlurane flow is regulated independently of total fresh gas flow, the adjustment error can result in too low a concentration of compound gas O.sub.2. In addition, the concentration changes upon adjusting the fresh gas flow. Desphlurane is supplied in glass flasks and the vaporizer is filled just like with conventional inhalation liquids. Prior to filling the container, the liquid supplied in flasks must be cooled to below the boiling point and it must be heated prior to using a vaporizer, which leads to a delay at the start of operation. Therefore, a vaporizer described in the cited Patent includes two parallel vaporizers. The mechanisms designed for traditional vaporizers for securing a minimum oxygen content are not applicable to a described type of vaporizer and, thus, without novel safety mechanisms the apparatus may produce fresh gas mixtures hazardous to a patient.
Separate safety mechanisms are in-built in methods and apparatus described in British Patent application No. 2239807. The apparatus includes an internal pressure regulator for adjusting the anaesthetic vapour supply pressure in patient circulation so as to conform with the prevailing fresh gas flow. By virtue of the apparatus, it is not possible to supply a minor fresh gas flow with a major anaesthetic vapour flow. The prevailing pressure and chemical conditions set a regulator with strict structural requirements: With a minimum fresh gas flow, typically less than 0.5 1/min, the pressure to be regulated is extremely low and, thus, even the mass of a regulator diaphragm may create a problem the same way as the spring forces and kinetic friction of a diaphragm. The chemical environment excludes most rubber types from being used as diaphragm materials. Even the remaining types or qualities have problems relating to deformation and fatigue. On the other hand, in metal diaphragms the spring forces are significant.
The regulator problems are avoided in a method and apparatus described in British Patent application No. 2239806. It describes various methods for preventing the pressure of an anaesthetic vapour produced by boiling liquid from discharging into patient circulation or for preventing the boiling which causes the generation of pressure. In the former case, the pressure of boiling liquid is vented to environment, which is a very poor alternative both in terms of economy, labour hygiene and environment. In the latter case, the liquid temperature is actively maintained below the boiling point by cooling the liquid container by means of a Peltier element. A drawback in activated cooling is a high power demand and a long starting delay, in case the liquid has heated to above the boiling point e.g. during the storage of a vaporizer. Another potential problem is also the dissipation of waste heat generated in cooling. The efficiency of a Peltier element is appr. 30% and, thus, in addition to the amount of heat transferred out of the liquid, there will be a double amount of waste heat produced by the transfer element.